A 60-kDa protein from rabbit reticulocytes specifically recognizes the capped 5' end of beta-globin mRNA

The binding of proteins from rabbit reticulocyte lysate to in-vitro-generated beta-globin mRNA and its defined segments was investigated using ultraviolet-cross-linking experiments as well as gel-retardation assays. Under stringent conditions, only three proteins (72, 60 and 50 kDa) were found associated with full-length beta-globin mRNA at different positions. The 72-kDa protein is most likely the poly(A)-binding protein and binds, as expected, to the poly(A) tail, whereas the 50-kDa protein exhibits affinity for the trailer region of beta-globin mRNA. The binding region of the 60-kDa protein is located at the 5' end of beta-globin mRNA. The interaction of this protein is dependent on the presence of the 5' cap structure, as indicated by competition experiments using an uncapped beta-globin-mRNA leader segment. Further competition experiments with beta-globin mRNA, deleted in part in the leader region, suggest that, besides the cap structure, certain sequence elements are necessary for the interaction of the 60-kDa protein and the beta-globin mRNA leader.